1. Field of the Invention
This invention relates to an improved process for the non-degenerative distillation of C.sub.6 -C.sub.24 -fatty acids, of the type obtained by splitting natural fats and oils or by synthetic processes, such as for example the oxidation of paraffins, by thermally drying the crude product under reduced pressure, subjecting the dried and heated crude product to fractional evaporation in falling-film evaporators, optionally in the presence of superheated steam, and depositing the various vapor fractions in condensers.
2. Statement of the Related Art
In addition to a mixture of free fatty acids, fatty acids derived from the hydrolytic splitting of natural fats or of oils on an industrial scale still contain unsplit fatty acid glycerides and substances of the type which accompany fats, such as sterols, phosphatides, polymerized fatty acids and other decomposition products, and also further impurities. In order to remove the undesirable impurities, which are partly responsible for undesirable discoloration, poor color stability and an unpleasant odor of the products, split fatty acids are subjected to distillation.
The distillation of fatty acids has long been carried out on an industrial scale. The first fatty acid distillation units consisted of retorts directly heated by smoke gases and operated under normal pressure. Since the crude fatty acids to be distilled were exposed to very high temperatures, resulting in partial decomposition or polymerization, the distillates obtained were more or less heavily discolored and, in addition, had an unpleasant odor. The quality of the distillates thus produced no longer satisfies current requirements.
In order to obtain distillates of high quality from crude split fatty acids, the distillation process has to be carried out at low distillation temperatures and, hence, under high vacuum with as short a residence time of the crude product to distillation as possible. On an industrial scale, distillation is generally carried out at 2 to 10 mbars and at temperatures of up to about 260.degree. C. (Ullmanns Enzyklopadie der technischen Chemie, Vol 11, pages 533 et seq, Verlag Chemie Weinheim, 1976).
Oils and fatty acids can be purified and decolored by adsorption processes. The most widely used adsorbents are natural and/or activated Fuller's earths. Active carbon is also used in some cases. The disadvantage of these adsorption techniques is that the bleaching materials used are relatively expensive and absorb a certain quantity of the product fatty acids which, in some cases, drastically reduces the yields.
Among the distillation-based purification processes, in almost all of which steam is now added to a greater or lesser extent to reduce the partial pressure and, hence, the boiling point of the fatty acids, there are a number of processes which use further additives during distillation. U.S. Pat. No. 2,583,028 describes the use of boron trifluoride, U.S. Pat. No. 3,833,629 describes the use of aromatic carboxylic acids, Japanese published Patent Application No. 7408-78 and U.S. Pat. No. 3,471,536 describe the use of aromatic or aliphatic amino compounds and British Patent No. 2,032,918 describes the use of reducing metals and hydroxides thereof. All these processes, which are based on separation of fatty acids by distillation in the presence of the above-mentioned additives, are attended by the disadvantage of more or less heavy product losses. In addition, corrosion problems occasionally affect those parts of the apparatus which come into contact with the crude mixtures at elevated temprature.